1. Field of the Invention
The present invention relates to a backlight assembly and a method of driving the same.
2. Description of the Related Art
Generally, a display device such as a liquid crystal display (LCD) device comprises a backlight assembly that provides a display panel with light to display an image in a dark place. A light source used in the backlight assembly may comprise a cold cathode fluorescent lamp (CCFL) and a light-emitting diode (LED).
In general, when power is supplied to the backlight assembly, a hold-type backlight assembly that continuously emits a backlight, regardless of the image that is displayed in the display panel, has been mainly used. However, recently, in order to decrease the power consumption and increase the contrast ratio of an image, a dimming type backlight assembly that adjusts the luminance of a backlight in accordance with the luminance of an image has been developed.
Methods of dimming a backlight comprise but are not limited to zero-dimensional (0-D) dimming method, one-dimensional (1-D) dimming method, two-dimensional (2-D) dimming method, and the like. According to the 0-D dimming method, the luminance of the display image is adjusted by the total screen. According to the 1-D dimming method, the luminance of the display image is adjusted by lines. According to the 2-D dimming method (or local dimming method), the luminance of the display image is adjusted by a small area of the display image.
In case of the CCFL, the 0-D and 1-D dimming methods are adapted, but the local dimming method is not adapted. The LED has characteristics, for example, such as low power consumption, small size, light weight, and the like, in comparison with a backlight assembly such as CCFL, therefore LEDs are generally adapted in a backlight assembly for an LCD device. That is, the local dimming method may be adapted in the LED.
In case of the local dimming method, LEDs corresponding to black areas of an image are turned-off, and LEDs corresponding to the other areas emit light in correspondence with the luminance of the image so that the light-emitting quantity is adjusted.
A driving current of a direct type or a pulse-width-modulation (PWM) type may be applied to the LED. In order to enhance color reproducibility and to emit light at uniform wavelengths, the driving current of the PWM type is used in the LED.
When a local dimming method is performed through the PWM method, the current quantity that flows in the LED is adjusted by the PWM dimming signal. The luminance of the LED is determined by the total current quantity that flows to the LED by varying PWM duty cycle when amplitude of a pulse current is fixed. Thus, a light-emitting quantity may be adjusted in accordance with a position of an image displayed in a display panel.
In the predetermined area corresponding to the predetermined image, it is often required that an image with high clarity and high luminance is displayed. Thus, a high luminance backlight assembly is required in the predetermined area, which is higher than the maximum luminance that may be obtained by the PWM duty cycle.